The Middle Devonian lacustrine sediments of Orkney, off the northeast Scottish mainland, are composed largely of the Lower and Upper Stromness Formations and overlying Rousay Formation. These three formations have been subdivided and defined by vertebrate biostratigraphic biozones with recent division of the Rousay Formation into three further units based on characteristic fish fossils. The division of the Rousay Formation has enabled a map to be constructed of the solid geology of the island of Westray, Orkney, based on fish identification, detailed logging of sedimentary cycles throughout the Rousay succession, parameters of divisional boundaries, and a survey of faults marking sinistral transtensional movement parallel to the Great Glen Fault. Post-Carboniferous shortening and basin inversion led to uplift, folding and reactivation of normal faults as reverse faults, to form a positive strike-slip flower structure in Westray. A suite of Permian igneous dykes intruded across Orkney include three minor offshoots in Westray. The resulting map is the first to make use of biostratigraphic units within the Rousay Flagstone which are now regarded as Members.